The present invention relates to a calibration system for chemical sensors. More specifically, the present invention relates to a pressure actuated calibration device which delivers a metered dose of calibrant to the atmosphere from a dosing chamber.
Industrial manufacturing, processing and storage facilities such as chemical plants, refineries and shipping terminals typically include a vast network of piping systems for transporting the raw or finished products through the facility. Such piping systems necessarily include a number of valves for controlling the flow of material through the facility.
Many of the products handled in the aforementioned plants are hazardous volatile organic compounds (VOC""s). Unfortunately, the valves used to control the flow of material through the plants typically experience a certain amount of undesired leakage referred to as xe2x80x9cfugitivexe2x80x9d emissions. Fugitive emissions, which are regulated by the Environmental Protection Agency (EPA), frequently occur around the packing between the valve stem and the body of the valve. These fugitive emissions must be monitored in order to comply with EPA emission regulations. Accordingly, leak detectors are placed near the valves, usually adjacent to the leak prone valve stems or other non-point sources, in order to monitor the leakage rate.
In order to obtain accurate readings, the leak detectors must be calibrated on a periodic basis, which typically must be accomplished from a remote location. One method of calibrating such leak detectors is to eject a small quantity of calibrant adjacent to the leak detector. The detector reading is then compared to a standard based on empirical data or a look up table, and the detector is adjusted accordingly.
In accordance with a first aspect of the invention, a leak sensor calibration device comprises a reservoir for storing a calibrant, a conduit in flow communication with the reservoir, an outlet nozzle, an air supply source, and a valve mechanism. A portion of the conduit defines a dosing chamber for storing a measured quantity of the calibrant, and the outlet nozzle is in flow communication with the dosing chamber. The air supply source is adapted to pressurize the measured quantity of calibrant stored in the dosing chamber. The valve mechanism is adapted to eject the measured quantity of calibrant stored in the dosing chamber to the atmosphere through the outlet nozzle.
In further accordance with a preferred embodiment of the invention, a flow restrictor is disposed between the dosing chamber and the reservoir. The flow restrictor may be a bi-stable check valve. Alternatively, the conduit may include a portion adapted to hold a second quantity of calibrant, with the second quantity of calibrant being greater than the measured quantity of calibrant. The conduit portion is disposed between the dosing chamber and the reservoir and thus forms a pneumatic restrictor.
The valve mechanism preferably includes a first remotely operable valve disposed between the dosing chamber and the outlet nozzle. The valve mechanism preferably also includes a second remotely operable valve disposed between the air supply source and the dosing chamber. The first valve is shiftable between a closed position and an open position. When in the closed position, the first valve isolates the dosing chamber from the outlet nozzle. The second valve is shiftable between a closed position and an open position. When in the closed position the second valve isolates the air supply source from the dosing chamber. A check valve may be disposed between the dosing chamber and the reservoir, with the check valve being shiftable between an open position and a closed position. The check valve is shiftable toward its closed position in response to movement of the second valve toward the open position.
The first and second valves are preferably electrical remotely operable valves, and are operably connected to a controller for remotely operating the first and second valves. Preferably, the controller is adapted to move the first valve to the open position for a first predetermined time interval, and to move the second valve to the open position for a second predetermined time interval. Still preferably, the second time interval is less than the first time interval and occurs during the first time interval. The first time interval may be approximately 50 milliseconds, while the second time interval may be approximately 10 milliseconds. As a precondition, the controller may move the second valve to the open position for a third predetermined time interval prior to the first and second time intervals. This precondition refills the dosing chamber more effectively.
In accordance with a second aspect of the invention, a leak sensor calibrating device for delivering a metered quantity of vaporized material to the surrounding atmosphere comprises a reservoir for storing the material, an outlet nozzle, a conduit providing flow communication between the reservoir and the outlet nozzle, a pressure source, and a valve system adapted to communicate the pressure source to the dosing chamber. The conduit includes a first portion defining a dosing chamber for storing the metered quantity of the material and also including a second portion disposed between the dosing chamber and the reservoir. The conduit second portion is adapted to impede the flow of the metered quantity back toward the reservoir. By operation of the valve system, the device ejects the metered quantity stored in the dosing chamber through the outlet nozzle to the atmosphere.
According to yet another aspect of the invention, a leak sensor calibrating device includes a reservoir for storing a fluid calibrant, and a conduit in flow communication with the storage reservoir. The conduit terminates in an outlet nozzle and includes a central portion defining a dosing chamber for storing a measured quantity of the fluid calibrant. The device also includes a pressure source for pressurizing the measured quantity stored in the dosing chamber, a valve system for isolating the dosing chamber from the surrounding atmosphere, and a control system operatively connected to the valve system.
In further accordance with a preferred embodiment, the reservoir may be adapted to contain a liquid analyte calibrant, and the dosing chamber may be sized to house therein a volume in the range of two (2) microliters. The reservoir may also be adapted to house therein an analyte calibrant in a vapor phase, in which case the dosing chamber may be sized to house therein a volume in the range of five hundred (500) microliters.
According to a still further aspect of the invention, a device is provided for use on a process system having a fluid material flowing through a conduit, such that the device will eject a measured quantity of the fluid material toward a sensor. The device comprises a port in flow communication with the conduit, with a portion of the port defining a dosing chamber for storing a measured quantity of the fluid material. An outlet nozzle is in flow communication with the dosing chamber, and an air supply source provides pressure to the measured quantity of calibrant stored in the dosing chamber. A valve mechanism is provided and is adapted to eject the measured quantity of calibrant stored in the dosing chamber to the atmosphere through the outlet nozzle. Accordingly, the constituency of the fluid material may be determined by the sensor.
Further advantages and features of the present invention will become evident to those skilled in the art upon a reading of the following description.